tautomerism

where the isomers (called tautomers) are readily interconvertible; the atoms connecting
the groups X, Y, Z are typically any of
C,
H,
O or
S, and G is a group which becomes an electrofuge or nucleofuge during isomerization. The commonest case, when the electrofuge is
H+, is also known as 'prototropy'. Examples, written so as to illustrate the general pattern given above, include:
Keto-enol tautomerism, such as:

The grouping Y may itself be a three-atom (or five-atom) chain extending the conjugation, as in:

The double bond between Y and Z may be replaced by a ring, when the phenomenon is
called ring-chain tautomerism, as in: